The preparation of water-absorbent polymers is, for example, summarized in “Modern Superabsorbent Polymer Technology” F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, or in Ullmann's Encyclopedia Industrial Chemistry, 6th ed. vol. 35 pp 73-103. The preferred preparation process is the solution or gel polymerization. When using this technology usually a monomer mixture is prepared which is discontinuously neutralized and then transferred into the polymerization reactor and is then discontinuously or continuously polymerized to form a polymer gel which is in case of a stirred polymerization comminuted. The polymer gel is subsequently dried, ground and sieved and optionally a surface treatment is applied.
Methods of continuous polymerization are, for example, described in WO-A-01/38402, WO-A-03/004237, WO-A-03/022896 and WO-A-01/16197.
Since the solution or gel polymerization is a free radical polymerization process this process is susceptible to the presence of oxygen. In free-radical polymerization oxygen is known to inhibit the polymerization reaction. Especially in continuous polymerization processes the presence of oxygen can create numerous problems since it makes the controlled start and progress of the polymerization difficult due to inhibition and chain termination reactions. Thus, in continuous polymerization reactions in the presence of oxygen it will be very difficult to control the radical polymerization and thus the desired properties of the final superabsorbent polymer. Furthermore it is known that the monomer solution is saturated with oxygen and that by feeding the monomer solution into the reactor an undesired high amount of dissolved oxygen is introduced into the polymerization process.
Thus, there were numerous attempts in the prior art to remove the dissolved oxygen from the monomer solution prior to the start of the free-radical polymerization reaction. In addition, measures were taken to conduct the free-radical polymerization reaction in an inert atmosphere.
U.S. Pat. No. 4,656,232 discloses a method for the polymerization of α,β-ethylenically unsaturated monomers to produce a superabsorbent resin by forwarding the aqueous monomer solution and an inert gas, each in the form of a continuous flow, toward the site of polymerization, spouting either of the two flows or fluids through a nozzle parallelly into the other flow thereby creating forced contact between the aqueous monomer solution and the inert gas and effecting substantial removal of dissolved oxygen from the aqueous monomer solution before the aqueous monomer solution reaches the site of polymerization. The aqueous monomer solution from which the dissolved oxygen has been removed is then admixed with the polymerization initiator and then subjected to polymerization in an atmosphere of inert gas. Inert gas is exemplified by nitrogen, carbon dioxide, argon, methane and ethane. By performing that process the concentration of dissolved oxygen in the aqueous monomer solution can be reduced from 7 ppm to 0.1 ppm.
WO-A-01/38402 relates to a continuous process for the preparation of a superabsorbent polymer. In this process the dissolved oxygen is removed from the monomer solution prior to transfer of the monomer solution into the reactor by directing an inert gas either cocurrently or countercurrently through the monomer solution whereby an appropriate admixing between the liquid phase and the gas phase can be achieved by valve, static or dynamic mixers or by a bubble column. The thereby inertized monomer solution is directed together with an inert gas stream through the reactor. Consequently, WO-A-01/38402 discloses the use of inert gas in two different stages of the process. First dissolved oxygen is removed from the monomer solution by applying an inert gas stream and then second the monomer solution is directed through the reactor together with an inert gas stream. Preferred inert gases are noble gases, like argon, carbonmonoxide, carbondioxide, sulfurhexafluoride, or mixtures of those gases. Only with respect to creating an inert gas stream within the reactor the possibility of creating the inert gas partially or completely by chemical reaction in the reactor, i.e. the mixing kneader, is disclosed. But in WO-A-01/38402 the use of nitrogen as inert gas when inertizing the monomer solution, as well as for the inert gas stream through the reactor is used. This is also evident from the examples in WO-A-01/38402.
WO-A-03/022896 discloses with specific reference to the examples the deoxygenation of the monomer mixture with a nitrogen stream creating a bubble column. The use of carbonates is only disclosed as a neutralizing agent in order to adjust the desired degree of neutralization of the monomer solution.
EP-B-688 340 with reference to the examples discloses a process for the preparation of superabsorbent polymers by preparing a monomer mixture in a beaker and thus under ambient atmosphere. Neutralization is performed by adding a solution of sodium carbonate in water with stirring. Thus, when preparing the neutralized monomer solution the aqueous solution is still saturated with dissolved oxygen. In the examples, in order to remove the dissolved oxygen, the monomer mix is sparged with nitrogen for one hour. Thus, the carbonate is used solely for neutralization purposes without any effect on the removal of dissolved oxygen.
From EP-A-827 753 a process for producing a water-absorbent resin capable of fast water absorption is known. The purpose of that process is to produce a sponge-like gel in order to obtain a fast water absorption. This is achieved by polymerizing a foam, i.e. a monomer solution containing dispersed inert gas bubbles. Although described in the general part of the specification as an optional component all the examples in that reference contain an emulsifier in the aqueous monomer solution in order to stabilize the foam. Furthermore, in the examples first the monomer solution is deoxygenated and then inert gas, especially nitrogen bubbles, are formed. Consequently, the inert gas bubbles in the foam that is subjected to polymerization are substantially free of oxygen which is in line with the teaching that polymerization is conducted in presence of the dispersed inert gas bubbles. The presence of oxygen in the dispersed inert gas bubbles would create the above problems related to inhibition and chain termination.
According to the teaching of the above discussed prior art references the aqueous monomer solution is deoxygenated prior to the start of the free-radical polymerization reaction. Thereby a considerable effort has been taken to ensure a preferably complete deoxygenation. According to the teaching of the prior art complicated mixing devices are necessary to ensure intimate mixing between the inert gas and the aqueous monomer mixture. A similar teaching is disclosed in U.S. Pat. No. 5,314,420.
Another problem of complete deoxygenation of the monomer mixture is that prior to the controlled start of the radical polymerization reaction, due to the lack of any inhibitor, it is difficult to avoid premature start of the polymerization within the monomer mixture, for example, in transfer lines. The problem is particularly pronounced in continuous polymerization processes since premature start of the polymerization reaction will result in the formation of gel prior to the entry of the monomer solution into the reactor resulting in fouling and blocking of transfer or off-gas lines. Especially in continuous processes this will influence flow rates and the whole process has to be periodically shut down for cleaning purposes.
WO-A-2007/028748 addresses this problem and suggests to include into the monomer solution 0.001 to 0.016 wt % of a polymerization inhibitor and that at least 50 volume percent of the used inert gas for inertization of the monomer solution is transferred together with the inertized monomer solution into the polymerization reactor. As polymerization inhibitor hydroquinone half-ethers and tocopherols are disclosed. Furthermore, it is described in WO-A-2007/028748 that the amount of inert gas used for inertization of the monomer mixture is considerably reduced compared to the amount known from the prior art.
As theory of the function of the proposed measures in WO-A-2007/028748 the following explanations are given. Due to the small amount of inert gas the monomer solution still contains more dissolved oxygen compared to the prior art which together with the inhibitor results in an increased inhibition of the monomer solution prior to entry into the polymerization reactor. In addition, the oxygen partial pressure in the gas phase is relatively high, so that oxygen during transport of the monomer solution to the polymerization reactor is still present for inhibiting purposes and due to the large interface area consumed dissolved oxygen can be supplemented by diffusion from the gas phase into the liquid phase to ensure sufficient inhibition during the transport of the monomer solution. In the reactor diffusion of oxygen from the gas phase to the liquid phase during the polymerization reaction is reduced due to the considerably reduced interface. But this clearly implies that oxygen is still present together with the added polymerization inhibitors within the reactor and thus during the free-radical polymerization reaction which has disadvantages, as discussed above. Thus, most of the prior art references teach to remove oxygen as completely as possible from the monomer solution prior to introduction of the monomer solution into the reactor in order to avoid any unwanted inhibition or chain termination reactions during the polymerization thereby creating problems of premature start of the polymerization reaction during transfer of the monomer mixture to the reactor. In WO-A-2007/028748 this problem is addressed by adding a polymerization inhibitor and by incomplete inertization of the monomer mixture but this technology has the disadvantage that considerable amounts of oxygen as well polymerization inhibitors are then present during the polymerization in the reactor.
Consequently, there is still a need for a continuous process for the production of superabsorbent polymers wherein premature start of the polymerization prior to entry of the monomer mixture into the reactor is minimized or avoided without adversely affecting the polymerization reaction by introducing too high amounts of oxygen or other inhibitors into the reaction mixture. According to another object of the present invention this goal is to be achieved without complicated devices in an economic way.